An optical phased array or beam former comprises a plurality of light sources arranged in an array and provides for controlling the direction of a beam of light that is formed by the light emitted from the plurality of light sources, by controlling the phase of the light from each light source. Alternatively, an optical phased array can also provide for receiving light from a controlled direction by controlling the phase of the light received on each light receiver of an array of receivers.
Known 2-D optical phased array control the phase of light waves transmitted by, or reflected from, a two-dimensional surface by means of adjustable surface elements. By for example dynamically controlling the optical properties of a surface on a microscopic scale, it is possible to steer the direction of light beams, or the view direction of sensors, without any moving parts. Phased array beam steering can be used for optical switching and multiplexing in optoelectronic devices, and for aiming laser beams on a macroscopic scale. A two-dimensional optical phased array or beam former can for example be used in an infra-red counter measures (IRCM) system, for directing a high-power beam of light at the optical aperture of a threat to dazzle and jam its optical seekers. Optical phased arrays can therefore improve the survivability of military and commercial platforms under attack from threat munitions and missiles that may be guided by a variety of electro-optic (EO) and infrared (IR) seeker types, such as semi-active laser (SAL) designator sensors and EO/IR imagers that sense one or multiple wavelength bands, with these seekers often sharing the same optical aperture. For at least these reasons, two-dimensional optical phased arrays are of interest to providers of military and commercial aircraft, as well as to any commercial developer of IRCM systems for military and civilian applications.
A two-dimensional optical phased array can also be used in compact laser radar systems or in LIDARs. Such systems can be used as altimeters for aircraft (including rotorcraft). Such systems also are envisioned for some automobiles. An advantage of a phased array is its large field of regard and fast beam steering, which means that one phased array can take the place of 4 to 10 or more small mechanical or micro-electro-mechanical (MEM) beam steerers.
The publication “Spatial optical beam steering with an AlGaAs integrated phased array,” (Applied Optics, vol. 32, pp. 3220-3232, 1993., by F. Vasey, F. K. Reinhart, R. Houdre, and J. M. Stauffer) describes a beam steering approach based on the use of integrated AlGaAs waveguide arrays on a GaAs substrate, in which each array element is a tunable phase shifter. The phase tuning is achieved via a linear electro-optic effect in the material by forming a heterojunction barrier with a low resistivity transparent conductor (indium-tin-oxide) cladding layer. Diffraction gratings patterned by electron-beam lithography couple light into and out of the device. Phasing is achieved electro-optically with voltages applied through indium tin oxide/AlGaAs Shottky junctions. Discrete beam steering was demonstrated with a 43-element rib waveguide array at an 850-nm wavelength. A sawtooth electrode kept the device length short and the electrode surface small. Continuous deflection over a ±7.2 mrad range at a 900-nm wavelength was reported. A set of seven sawtooth and offset electrodes allowed addressing of points within this range. The beam had a width of 1.5 mrad, and the maximum modulation voltage is −8.5 V.
A problem with this approach is the rather limited phase delay achievable with the described tunable waveguides due to the weak electro-optic effect of AlGaAs. Another problem with this approach is that waveguide lengths of more than 3 mm and large control voltages (7.2 volts) are required just to steer the output beam by 10 mrad at 0.85 μm wavelength. The waveguides in the array have a spacing of 12 μm, which is 14 times larger than the wavelength of the light. Thus, the far-field pattern generated with this approach has prominent grating lobes.
As taught in the publication “Suppression of sidelobes in the far-field radiation patterns of optical waveguide arrays”, by J. H. Abeles and R. J. Deri (Applied Physics Letters, vol. 53, pp. 1375-1377, 1988), it is possible to avoid producing grating lobes by using non-uniform spacing between the emitters of the array.
The publications: “Integrated optical phased array based large angle beam steering system fabricated on silicon-on-insulator,”, by D. Kwong, Y. Zhang, A. Hosseini and R. T. Chen (Proceedings SPIE Vol. 7943, p. 79430Y, 2011) and “1×12 Unequally spaced waveguide array for actively tuned optical phased array on a silicon nanomembrane” by D. Kwong, A. Hosseini, Y. Zhang, and R. T. Chen (Applied Physics Letters, Vol. 99, 051104, 2011), disclose an optical 1-D phased array with non-uniform emitter spacing that uses thermo-optic phase shifters fabricated in silicon waveguides (a thermal dependency of the Refractive Index is called the thermo-optic effect).
FIG. 1 shows such an optical 1D phased array 10 with non-uniform emitter spacing, which uses a plurality 12 of substantially parallel thermo-optic phase shifters fabricated in silicon waveguides. Each thermo optic phase shifter comprises a thermo-optic section 13 having a heater element with a control pad 14. All of the thermo optic phase shifters 13 of the plurality 12 receive their light from a single light input 16 through a coupler section 18. The output of the thermo-optic section 13 of each phase shifter is coupled to an S-bend passive phase shifter section 20 that allows positioning the outputs of the phase shifters with a spacing selected to form a non-uniform array of outputs 22, to ensure that the grating lobes associated with those sub-arrays occur at different angles.
FIG. 2 shows in detail the layout of the plurality 12 of phase shifters 13 of FIG. 1 (without showing the control pads 14). Thermo-optic phase shifters 13 have each a length of substantially 0.5 mm and can tune the phase shifts over 0-2 rt. The time needed for switching the phase between 0 and π is less than 10 μsec for these compact silicon waveguides. A metal heater strip (not shown) placed above the waveguide is used to change the temperature of the waveguide. The array shown in FIGS. 1 and 2 allows steering a 1.55 μm wavelength output beam by 30° while applying a power of 21 mW per waveguide channel, for an array having 12 separately controlled waveguide channels.
FIG. 3 is a photograph showing a cross section of one thermo-optic phase shifter 13 of the array of FIGS. 1 and 2. Thermo-optic phase shifter 13 comprises a silicon waveguide 24 formed on top of a SiO2 layer/box 27, at the bottom of a trench 26 cut through a PECVD SiO2 layer 28 formed on top of SiO2 layer 27. A PECVD SiO2 plug 30, formed on top of silicon waveguide 24 such that the edges of plug 30 are isolated from the edges of trench 26 by air gaps 32, is covered by a heater 34, for example a strip of Cu/Au. SiO2 layers 27 and 28 can comprise air regions (not shown) surrounding each thermo-optic phase shifter 13, to isolate them thermally from each other.
It is noted that the array illustrated in FIGS. 1 and 2 actually comprises several sub-arrays for which the element-to-element spacing within each sub-array is uniform. On another hand, the different sub-arrays have different element-to-element spacing that are selected to ensure that the grating lobes associated with those sub-arrays occur at different angles.
It is also known to have, alternatively to phase shifters using thermo-optic materials, phase shifters using electro-optic materials. Such phase shifters can use optical waveguides fabricated from GaAs/AlGaAs materials and make use of the quantum confined Stark effect (QCSE) in a multiple quantum well (MQW) structure to achieve a stronger voltage-controlled phase modulation. For example, with a phase-shift section length of 1.5 mm, a voltage change of approximately 4 Volts is needed to change the phase by 2π for 0.87 μm wavelength light.
The inventors have noted that all of the known phased arrays use optical phase shifters capable to operate at only one wavelength at a time. However, there exists a need for an optical phased array capable of simultaneously emitting or receiving directional light beams having different wavelengths.